Download Chapter 6 - Section 1-The Chemical Context of Life

Chapter 6
The Chemical Context of Life
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: A Chemical Connection to Biology
 Biology is a multidisciplinary science
 Living organisms are subject to basic laws of
physics and chemistry

Example: Ants - Species Myrmelachista
o Kill non-host trees by injecting formic acid into
their leaves
o They maintain an environment that their colony
can survive and thrive
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
 Life can be organized into a different
structural levels.
Organism
Organ System
Tissues
Cellular
Molecular
Atomic
 At each level additional distinct
properties appear.
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Matter consists Elements & Compounds
 Organisms are composed of matter
 Matter is anything that takes up space and has mass

Matter is made up of elements and compounds
 ELEMENTS & COMPOUNDS
 Element: a substance that cannot be broken
down into other substances by chemical
reactions.
 Compound: is a substance consisting of two or
more elements in a fixed ratio
o A compound has characteristics different from those
of its elements
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
E.g. Table Salt
Sodium
Chlorine
Sodium
chloride
Essential Elements of Life
 About 25 of the 92 elements are essential to life
 96% of living matter

Carbon

Hydrogen

Oxygen

Nitrogen
 Remaining 4%

Phosphorus, Sulfur, Calcium, Potassium, Others
 Trace elements are those required by an organism in minute
quantities

Iodine & Iron
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Fig. 2-4
(a) Nitrogen deficiency
(b) Iodine deficiency
Concept 2.2: Structure of An Atom
Each element consists of unique atoms
Atom: Smallest unit of matter that still retains
the properties of an element
Atoms are composed
 Neutrons (no electrical charge)
 Protons (positive charge)
 Electrons (negative charge)
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Cloud of negative
charge
(2 electrons)
Atom
Proton
Charge
+ve
Neutron
Neutral
Electron
-ve
Electrons
Nucleus
Properties
Stays the same in an element & determines element
Varies in an element
Determine chemical properties & reactivity of an element
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Atomic Number and Atomic Mass
 Atoms of the various elements differ
in number of subatomic particles

Atomic number: Number of
protons in its nucleus

Mass number: Sum of protons
plus neutrons in the nucleus

Atomic mass: The atom’s total
mass, can be approximated by
the mass number
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Isotopes
 All atoms of an element have the:

Same number of protons

But may differ in number of neutrons
 Isotopes: Are two atoms of an
element that differ in number of
neutrons
 Radioactive Isotopes: Decay
spontaneously, giving off particles and
energy
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Some applications of radioactive isotopes in
biological research are:
 Dating fossils
 Tracing atoms through metabolic processes
 Diagnosing medical disorders
PET Scan – Detects location of intense
chemical activity
 Pt. injected with glucose labeled with
radioactive carbon
 Particles collide with electrons from
chemical reactions in the body
 The PET detects these hot spots”
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Chemical Bonds
 Interactions between usually result in
atoms staying close together, held by
attractions called chemical bonds
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Covalent Bonds
A covalent bond is the sharing of a pair of
electrons by two atoms
Covalent bonds can form between atoms of the
same element or atoms of different elements
In a covalent bond, the shared electrons count as
part of each atom’s valence shell
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 2-11
Hydrogen
atoms (2 H)
Hydrogen
molecule (H2)
 A molecule consists of two or
more atoms held together by
covalent bonds
 A compound is a combination of
two or more different elements
 A single covalent bond, or single
bond, is the sharing of one pair of
valence electrons
 A double covalent bond, or double
bond, is the sharing of two pairs of
valence electrons
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Ionic Bonds
Atoms sometimes steal electrons from their
bonding partners

An example is the transfer of an electron from
sodium to chlorine
After the transfer of an electron, both atoms
have charges
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Ionic Bonds
A charged atom (or molecule) is called an ion
 Those that gain electrons become
negatively charged
 Those that lose electrons are positively
charged
Na
Na
Sodium
atom
Cl
Cl
Chlorine
atom
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Na
Cl
Cl–
Na+
Chloride
Sodium
ion
ion
(an anion)
(a cation)
Sodium chloride
(NaCl)
A Cation is a positively charged ion
An Anion is a negatively charged ion
An Ionic bond is an electrical attraction between
an anion and a cation
Compounds formed by ionic bonds are called
ionic compounds, or salts
Ionic Bonds are weak
Salts, such as sodium chloride (table salt), are
often found in nature as crystals
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Fig. 2-15
Na+
Cl–
Hydrogen Bonds
A hydrogen bond forms
when a hydrogen atom
covalently bonded to one
electronegative atom is
also attracted to another
electronegative atom
In living cells, the
electronegative partners
are usually oxygen or
nitrogen atoms
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 2-16

+
Water (H2O)
+
Hydrogen bond

Ammonia (NH3)
+
+
+
Van der Waals Interactions
• If electrons are distributed asymmetrically in molecules or
atoms, they can result in “hot spots” of positive or negative
charge
• Van der Waals interactions are attractions between
molecules that are close together as a result of these
charges
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
• Collectively, such interactions can be strong,
as between molecules of a gecko’s toe hairs
and a wall surface
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
You should now be able to:
1. Identify the four major elements
2. Distinguish between the following pairs of
terms: neutron and proton, atomic number
and mass number, atomic weight and
mass number
3. Distinguish between and discuss the
biological importance of the following:
nonpolar covalent bonds, polar covalent
bonds, ionic bonds, hydrogen bonds, and
van der Waals interactions
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings